1. Field of the Invention
This invention concerns novel and improved grease compositions comprising a base oil, an EP agent and an amount of soap in excess of that required to thicken said composition to the desired grease viscosity.
The traction motor gear box which forms a part of the drive train for locomotives requires a high-viscosity grease with high film strength. In order to meet the specifications set by the users for such greases, manufacturers of formulations must incorporate film strength additives, or as they are generally known EP (Extreme Pressure) additives, in their greases. In addition, the amount and kind of soap thickener used in the greases must be selected to maintain a high viscosity in the product. Typical specifications for tractor motor gear box greases are shown in Table 1.
TABLE 1 ______________________________________ TRACTION MOTOR GEAR BOX SPECIFICATIONS FOR LITHIUM BASE GREASES General Electric General Motors ______________________________________ Brookfield Viscosity @ 200.degree. F. #4 Spindle at 20 RPM 7000 cP #3 Spindle at 4 RPM 5000-10000 cP 4 ball E.P. Test Load Wear Index 50 kgf (min) 40 kgf (min) Weld Point 300 kgf (min) 200 kgf (min) Timken 40 lbs, OK Load -- Base Oil Viscosity 1500 SSU 1500-2000 SSU at 210.degree. F. at 210.degree. F. ______________________________________
Typically, grease formulators have had difficulty in maintaining the desired viscosities of greases during their use in tractor motor gear boxes. A testing program was set up to determine, if possible, the reasons for the failure of greases to meet specifications and to find a solution to the problem or problems. In addition to failure of greases during use, problems have been encountered with substantial viscosity loss during shipment of greases in heated railway cars to prospective users.
The greases studied in the test program each contained a soap or soaps, one or more EP agents, and various base oils. One of the soaps used was a standard soap, lithium 12-hydroxy stearate. Also tested were greases containing complex soaps which were a mixture of lithium 12-hydroxy stearate and the lithium salt of adipic acid. The EP agents used in the greases were Lubrizol 6063, a sulfurized ester of an organic phosphate; Lubrizol 887, zinc dithiophosphate and sulfurized ester; Sulperm 110, a sulfurized synthetic sperm oil; and Hitec 2319, a sulfurized ester of an organic phosphate. Base stocks used in the greases included bright stock, 200 neutral paraffin, and Duosol extract. Greases were heated in ovens at 200.degree. F. and 300.degree. F. to simulate rail car grease shipments during the summer and winter months, respectively, and also to simulate the temperatures expected in the traction motor gear boxes. Several greases containing standard soap and base oil, but no EP additive, were heated at 200.degree. F. and at 300.degree. F., for a total of 168 hours. These greases, which had initial Brookfield viscosities at 200.degree. F. varying from 20,000 to 40,000, showed no decrease in viscosity at either temperature for the time period tested. Similar greases containing EP agents showed a loss in viscosity at the same temperatures and over the same time period varying from as low as 20% to as high as 93%. Similar results were obtained with greases containing complex soaps.
The tests established that major reductions in viscosity of the greases occurred at temperatures at which the greases were transported and stored and at temperatures encountered during use in the traction motor gear boxes. The data also established that the reductions in viscosity encountered were due to reaction of the EP agents with soap dispersed in the grease. In some of the tests, the greases ultimately lost essentially all of their viscosity and became as fluid as the base oils. Such greases would quickly leak from a gear box and thus would be totally ineffective as lubricants.
According to this invention, stable greases comprising a soap, an EP agent, and a base oil are prepared in which the amount of soap used is in excess of that required to thicken the grease to the desired viscosity. When an oil insoluble EP agent is used, the soap is prepared in such a manner that the grease contains undispersed soap solids as well as dispersed soap. As the EP agent slowly reacts with the dispersed soap, undispersed soap solids then become dispersed in the grease through the shearing or milling action of the gears being lubricated. With the correct selection of the soap and EP agent and the amount of each of these materials, it is possible to provide a balanced system whereby sufficient dispersed soap is present at all times to maintain the desired viscosity of the grease.
When an oil soluble EP agent is used, the soap is prepared in such a manner that when it is combined with the base oil it will be substantially entirely dispersed in such oil. The soap is then contacted with the EP agent, which quickly reacts with a portion of the soap to neutralize or substantially neutralize the reactivity of the EP agent. The remaining soap dispersion is then combined with the base oil to form a stable grease.
2. The Prior Art
Japanese Pat. No. 60-173,097 to Idemitsu Kosan KK relates to a lubricating oil composition prepared by adding to a base oil a phosphoric acid or its amine salt and at least one compound of the fatty acid type additive.
German Pat. No. 1,909,804 to Norway discloses a lubricating composition comprising a lubricating oil, phosphate and a salt of a fatty acid wherein the ratio of the fatty acid to the phosphate is about 1:1 to 3:1 and the total amount of the phosphate and fatty acid salt is about a to 50% by weight of the whole lubricant.
British Pat. No. 830,887 to Esso Research & Engineering Company discloses a high temperature grease for use in anti-friction bearings comprising a lubricating oil, from 15 to 40 weight percent of a soap comprising sodium crotanate and the sodium salt of an unsaturated monocarboxylic acid containing 14 to 22 carbon atoms and 0.1 to 15.0 weight percent di- or tri-sodium phosphate, said crotanate, unsaturated sodium salt and phosphate having been heated to a temperature of from 300.degree. F. to 425.degree. F. for from 20 minutes to 4 hours while in admixture with at least a portion of said lubricating oil.